Ceramic cements are mixtures of water and reactive metal oxides that react to form a hardened mass. Cements are often used as adhesives to concrete. Portland cement, for example, is a mixture of water and calcined lime and silica that cures to form principal phases of di-calcium silicate and tri-calcium silicate. Portland cement has attractive handling and cost attributes, yet suffers from inconsistency of physical properties, relatively high viscosity, and slow curing rates. As a result, Portland cement is not well suited for pumping or spraying. In addition, Portland cement has poor adhesion to a Portland cement substrate. As a result when there is an interruption in forming a Portland cement body, a structural discontinuity results thereby precluding usage as a surface coating or patching.
Phosphate based cementitious materials address a number of limitations associated with Portland cement and are characterized by excellent strength and hardness properties and adhesion to most materials, including set cements, brick, metal, wood, most wood products, and asphalt. Phosphate based cementitious materials also have good chemical stability and compressive strength, and toughness superior to that of Portland cement. Moreover, phosphate based cementitious materials tend to set up with little or no open porosity and therefore can be used to form waterproof forms and seals.
A desirable phosphate based cementitious material has the characteristics of an adjustable set time, strength maintenance over time at operating temperature, and limited dimensional changes as a function of temperature. The production of an advantageous cementitious material is particularly problematic when the cementitious material is used as a refractory. In refractory cementitious material, the high temperatures experienced serve to enhance dimensional changes while facilitating undesired chemical reactions that are of little consequence at lower operating temperature.
Cementitious materials using a phosphate based binder to set the relative position of aggregate particulate, while effective in forming a variety of cementitious materials, have proven difficult in practice to obtain the properties theoretically achievable. In the case of calcium aluminate cements, the reaction of phosphate with a calcium source yields a calcium phosphate based binder. As the phosphate reagent such as phosphoric acid is typically provided in solution form, the kinetics and homogeneity of reactive calcium ions is a factor in determining cementitious material set kinetics and strength.
The prior art teaches the use of calcium monoaluminate (CA) and calcium dialuminate (CA2 or synonymously known as grossite) as dominant sources of calcium ions for the formation of calcium hydrogen phosphate binder, as detailed for example in U.S. Pat. No. 5,888,292. The propensity of calcium monoaluminate to hydrate is a significant contributor to the early set strength of phosphate cements, and reaction from this calcium source tends to be slow. Grossite based on a framework of aluminum tetraoxide tetrahedra imparts the more refractory nature on the resulting cement than CA with the cost of being slower to set and as a result more amenable to unreacted inclusions within the binder. These aluminum rich calcium aluminate bases as a calcium source tend to slow material set and incorporate aluminum rich inclusions that reduce the overall operating temperature of a cementitious material and material strength.
In order to obtain refractory cementitious materials at melting temperatures in excess of 2000° F. (1093° C.), resort is often made to magnesium phosphate binders produced through the reaction of magnesium oxide with a soluble phosphate or phosphoric acid. A problem associated with formation of a magnesium phosphate binder is a highly exothermic reaction associated with neutralization of phosphoric acid by magnesium oxide. Practically, reactions between phosphoric acid and magnesium oxide result in weak articles setting at room temperature and set times which are often too quick for proper handling. As a result, magnesium oxide is combined with an aluminum phosphate solution even though cost and working properties would otherwise favor the reaction of magnesium oxide with phosphoric acid.
Additional problems associated with phosphate cements include phase transformation from polyphosphates, anonymously known as hexaphosphates and/or metaphosphates, to orthophosphates. The phase transformation to orthophosphate results in a reduction in crush strengths in the range of from 1000° F. (538° C.) to 1500° F. (816° C.). As a result, the durability of such cements in furnaces operating in this temperature range is reduced.
Phosphate cements are also particularly susceptible to metal contaminants that readily form metal phosphates. Metal contamination is often associated with degradation of crushing and cutting tools used to process cementitious material precursors and refractory aggregates which are part of the formulation.
The reaction of metal with phosphoric acid inevitably releases gaseous byproducts that are either retained as voids within the cementitious material or percolate therethrough yielding low energy crack propagation pathways through the material.
Thus, there exists a need for phosphate cement additives for addressing the aforementioned limitations of existing phosphate cement binder. The ability to control set properties and strength facilitates the use of cementitious materials particularly as refractories.